The present disclosure relates generally to methods of appraising offshore reservoirs. More specifically, in certain embodiments the present disclosure relates to long-term, low-rate, intermittent floating hydrocarbon production systems for appraising offshore reservoirs and associated methods.
One objective of appraising recently discovered fields is to provide sufficient data so that reliable estimates of the costs, revenues, and risks associated with different field development scenarios can be made. After appraising the fields, an informed decision on how the opportunity may be best valued in the future (e.g., the selection of a field development plan, a divestment strategy, a phased development, or just holding the discovery pending nearby exploration) may be made. From a well and subsurface perspective, field appraisal aims at reducing uncertainty on the following points: the volume of hydrocarbons present, the production profile for various development scenarios, the expected initial well flow rates, the sustainable well flow rate, and the ultimate recovery per well.
Systems for appraising deepwater reservoirs typically follow one of three approaches to appraise deepwater reservoirs: (1) appraisal campaigns with multiple wells which may then be followed by short-term well tests, (2) long-term well testing, and (3) early production systems.
The first approach, while being the most common, may be the most costly due to the high drilling and completion costs. Very short term well testing (also known as drillstem well testing) is possible with this approach; however the information gathered by this approach is often limited to the vicinity of the well bore.
The second approach also tends to be costly, since it usually makes use of expensive mobile offshore drilling units to perform the well testing activity. Since the mobile offshore drilling units often do not have any significant gas storage and export capabilities, the length of time of the well test is also typically limited.
The third approach, which typically implies an oil processing rate capacity of at least 5,000 barrels a day, requires the installation of a gas export pipeline or a regulatory exemption to flare the produced gas throughout the exploitation of the reservoir by the production system. The capital cost of such a system tends to be high, especially when the gas pipeline scope is included. The alternative, long term flaring, is still costly, generally limited in terms of volumes and location, and not favorable from an environmental standpoint, restricting the usefulness of early production systems.
Low-rate well testing is relatively uncommon in deepwater developments. Such low rates may allow for an appraisal system comprised of a small floating system, with limited amounts of gas (as compressed natural gas) storage volumes, to be used intermittently. The use of such low-rate appraisal systems may be desirable because they would allow the use of intermittent floating hydrocarbon productions systems.